Technology already exists which permits the measurement of spectral radiant power from the earth's surface. Systems carried by an aircraft or orbiting satellite are used with great success in making determination of all sorts of earth conditions, one center for this work being at the Laboratory for Applications of Remote Sensing (LARS) at Purdue University, West Lafayette, Ind.
One useful function of remote sensing is to predict world crop conditions. One important parameter in making such a prediction is the determination of soil wetness in certain major agricultural regions of the world.
In the past, these soil wetness determinations have commonly been made on a gross basis based on soil color, but it was not thought that there was any good correlation between color and wetness such that accurate determinations could be effectively made by remote sensing.
At the present time, measurement of soil water in the field depends upon sampling a given location, both in area and depth of soil profile, at a given time or times. These samples are then used to estimate the water condition of an entire area. There is no known method, however, for rapidly analyzing a large number of samples over a large area as is possible using the method of this invention (which includes establishing spectral curves for the soils in question to facilitate rapid analysis). In fact, utilizing this invention, a continuous reading across a landscape could be achieved with an electromagnetic sensor of suitable type mounted on a satellite, aircraft, or even on a ground vehicle.
Soil moisture measurements have heretofore been aimed at two soil water variables; the soil moisture content and the magnitude of the soil water potential, which is the negative of the work that must be done to remove a unit amount of the relatively loosely held soil water.
Tension, however, is the most important soil water property as far as plant behavior is concerned. Most plants suffer from lack of sufficient oxygen in the root zone when the tension is less than 1/3 Bar (field capacity) and cannot take water into their roots from surfaces of soil particles at tensions higher than 15 Bar (wilting point). Since tension depends mainly on water film thickness on particle surfaces, measurements of tension do not vary with changes in water content due to changes in the total area of the wetted surface. Hence, moisture tensions do not vary with soil texture or structure as does water content. Because of this, measurements of tension do not change with soil texture and thus are a convenient and precise way of characterizing the soil moisture regime for many practical purposes. Thus the method of this invention, which provides a rapid and accurate way of estimating soil moisture tensions, is of practical value.
Methods for measuring soil moisture are either direct or indirect. Direct methods normally call for taking samples of soil and estimating the amount of water per unit weight or volume of that soil. A sample of moist soil is dried at 105.degree.-110.degree. C. and reweighed when cool. Shortcomings are that the method is laborious and time consuming. Samples of field soils must be brought in airtight containers to a laboratory where balances and an oven are available. Also, in studying moisture changes in field soils, samples need to be taken at frequent intervals and at each time the new values are needed because of changes caused by precipitation, evaporation, etc. Furthermore, it is hard to tell whether the loss is due to oxidation of organic matter or to drying. Consequently, efforts have been aimed at standardizing procedures to give reproducible results rather than absolute, unequivocal values. With forced air, ten hours are usually required for drying, while with only convection currents, twenty-four hours are usually required for drying.
Modifications have been suggested to improve or speed up the drying method. One method is to mix calcium carbide with the soil water to cause the water to react to form acetylene gas which evaporates. The moisture is then determined by weight difference. This method, while rapid, is not an accurate or satisfactory method. Freeze drying has also been used. In this method, the sample is frozen, the water allowed to sublimate, and drying is continued after most of the water is removed. This method, however, calls for several operations and calls for standardizing of time and temperature to get reproducible results.
Another method proposed for speeding drying is through partial removal of water from soil samples with alcohol. After evaporation, the alcohol is burned. Results approximate those of oven drying after several treatments and burnings. This method, however, is not accurate for high organic matter soils and generally is not as accurate as the standard oven drying method.
Infrared drying has also been used but does not provide for high precision. Here again, a sample has to be brought to a laboratory and carefully prepared and treated with a standardized technique. Water can also be removed by centrifuging the wet soil sample.
For years, attempts have been made to devise an indirect method for determining soil moisture content quickly and without removing a sample from the natural situation. All these methods have resulted, however, in some disturbance of the natural condition when the apparatus is installed and are limited to the site of the installation. In addition, these methods have proven to be difficult because of the influence of soil texture and structure and of the soil solution. Hysteresis, the greater energy status which exists for soil moisture as the soil is drying compared to when it is being wetted, has also been a problem.
The basic technique heretofore commonly utilized has been to use a porous absorber which comes to equilibrium with the soil moisture. The conductance between two electrodes placed in the ground and measured by some form of Wheatstone bridge has been proposed, and without interfering factors, the conductivity varies with moisture content. In actual practice, however, changes in content of soluble salts and the difficulty of securing good contacts between electrodes and soil have been found to cause errors.
A gypsum block cast around two electrodes has also been proposed. Such a block can be buried in the soil at any depth. More recently, a unit composed of a nylon fabric encased in perforated metal and containing two electrodes has been developed.
A tensiometer has also been developed for measuring the force by which water is held in the soil expressed in terms of centimeters in height of a column of water required to produce a force of equal magnitude. The tensiometer consists of a porous clay cup attached to a mercury manometer. Tensiometers have to be set up at definite positions but once installed provide a ready reading of the tension of the soil moisture within a limited range. The highest reading possible, however, is less than one atmosphere (approximately 1000 cm. of water). The relation between soil suction and water content is not single-valued, but is influenced by soil texture and structure. Generally, however, it is not deemed necessary to interpret soil water tension in terms of moisture content for the reasons brought out hereinabove.
Soil tensions can also be measured using a centrifuge, but as with air drying techniques, this requires bringing samples to the laboratory for special preparation and treatment.
Electrical and thermal conductivity and electrical capacitance have also been studied. Unfortunately, such measurements made directly in soil have not resulted in unique correlations with water content and hence have not come into general use. Variance results from uncertain contact between electrodes and the soil as well as soil heterogeneity.
Neutron absorption or neutron attenuation has not been widely used to determine moisture content because it is limited to locations near reactors. Also the equipment must be heavily shielded. The shielded neutron probe must be lowered into an aluminum or steel tube which was previously placed in the soil. Since this method has little effect on soil moisture content, it can be used repeatedly. It cannot be used effectively, however, on small volumes of soil.
Gamma ray attenuation techniques are not limited with respect to location, but the equipment is expensive and requires shielding and other safety features. This method needs a definite set up but once installed is precise when the soil has a constant bulk density. The soil, however, must be disturbed and moved to the laboratory. The method has been used in research on water movement in soils. A disadvantage is that to be used to measure absolute moisture constants, each soil to be studied has to be calibrated.